1. Field of the Invention
The present invention relates to metrology, and more particularly to line width and overlay metrology.
2. Description of Related
In the original atomic force microprobe, a very small probe tip with a submicron radius floats across a slowly undulating surface with displacements at the nanometer level. The tip is vibrated perpendicular to the surface and depending upon the distance from the surface, a change in resonance occurs due to the Van der Waal's force between the surface and the tip. The change in resonance is sensed with a laser heterodyne interferometer. The tip can be maintained either at a fixed height through a feedback loop or the change in resonance can be converted to a height signal. The sensitivity of the change in resonance allows accurate height measurements at the subnanometer level. The microprobe force range is in the range of a few tens of nanometers and in this mode of operation, the tip remains within the force range as the specimen is scanned using a piezo-electric scanning stage.
The principal drawback to this mode of operation is the limited resolution and accuracy of the measurements in the lateral x-direction due to tip geometry. Resolution is a function of step height or slope of the surface as shown in FIG. 1A and FIG. 1B. FIG. 1A shows a microprobe tip 17 of probe 19 following a slowly undulating surface 16. FIG. 1B tip 17 shows a nearly vertical step 18. The point corresponding to maximum probe force changes during the scan especially for the case of FIG. 1B. For nearly vertical edges it is limited to the radius of the tip at the step height. Current parabolic tungsten tips have a tip radius of approximately 0.5 micrometers and 0.1 micrometers and smaller have been fabricated in other materials.